hvac system shuts down periodically 92 blazer
 

just bought a 92 blazer.. the heat works fine but the ac made a loud squealing sound for awile (hasn't made the noise lately)and clicking noise and doesnt blow cold air and quite often the whole system will shut down when on heat or cold but come back on in 15 seconds er so. the whole ac system was recharged in o9.. but has never worked properly.. mechanic said they couldnt find a leak anywhere?. please help

Did the mechanic do a system performance test with a manifold gauge set? That's a pretty basic part of AC service. If so, what were your high and low side pressure readings?
There are seven operating conditions of an AC system, with condition 1 being normal operation. The remaining six result in insufficient cooling, and those are determined by the system pressure generated. You need to find out if your pressures are high, low, normal, etc.
I'm a bit curious as to what your mechanic did. How did he leak test the system? Did he use dyed refrigerant, a halogen sniffer, soap and water (I sure hope not on the last one). How long was it between the refrigerant charge and your mechanic looking for leaks? Have you had to replace any component of that AC system which required the system to be opened up? When your mechanic checked for leaks, did he evacuate the system and weigh the refrigerant he recovered?